1. Field of the Invention
The present invention relates to a process for producing grain oriented silicon steel sheet. In particular, the present invention is directed to a decarburization annealing process for improvement of magnetic characteristics and film characteristics by controlling the physical properties of the surface oxides layer which is formed in the step of decarburization annealing. The invention further relates to a novel silicon controlled sheet that results from the decarburizing step of the process.
2. Description of the Related Art
Grain oriented silicon steel sheets are used as soft magnetic materials mainly for iron cores of transformers and rotating electrical machines. They are required to have high magnetic flux density, small iron loss and small magnetostriction. High magnetic flux density is attained by highly aligning the crystallographic orientation by the process of secondary recrystallization. The aligned structure has the so-called GOSS orientation having a {110} surface in the steel sheet surface and an &lt;001&gt; axis of easy magnetization.
Iron loss includes both eddy current loss and hysteresis loss. Eddy current loss is influenced by the thickness and electrical resistance of the steel sheet and, in addition, by the film tensile force, magnetic domain width and crystal size of the steel sheet. On the other hand, hysteresis loss is influenced by the crystal orientation, purity, strain and surface smoothness. For the purpose of reducing hysteresis loss it is effective in particular to align the crystal orientation in the direction of the axis of easy magnetization. Generally, it is accepted in grain oriented silicon steel sheet to align the crystal orientation by secondary recrystallization to a so-called GOSS orientation of {110} &lt;001&gt;. Magnetostriction is reduced to a small value by alignment of the crystal orientation or by increase of the tensile force of the film.
Hence, it is very important for reduction of iron loss and magnetostriction to enhance the integration of GOSS orientation.
Grain oriented silicon steel sheets are produced from a grain oriented silicon steel slab which contains an inhibitor such as MnS, MnSe or AlN, required for secondary recrystallization. The slab is heated and subjected to hot rolling. Thereafter, annealing is performed as required. One time of cold rolling, or two or more times of cold rolling with intermediate annealing follows. This reduces the sheet thickness to the final value. Next, annealing is performed which functions both for decarburization and first recrystallization. Then an annealing separator comprising MgO or the like as the main component is coated on the steel sheet, which is subjected to high temperature finishing annealing to achieve secondary recrystallization.
The grain orientation inhibitor functions to direct the grain toward the GOSS orientation, selectively inhibiting the growth of grain in other orientations in the primary recrystallization structure. It is therefore indispensable for secondary recrystallization.
Two types of such inhibitors are known. One serves as a fine precipitate; examples of this type include AlN, MnSe and MnS. The other type induces grain boundary segregation; examples of this type include Sb, Sn, Nb and Te. The fine precipitate type is presently mainly used in the production of grain oriented silicon steel sheet. For success with an inhibitor of the fine precipitate type, it is important to disperse uniformly a necessary and sufficient amount in fine size because the inhibitor when dispersed uniformly inhibits the growth of primary recrystallization grain until secondary recrystallization takes place.
Forsterite (Mg.sub.2 SiO.sub.4) insulation film is often formed on grain oriented silicon steel sheets except in special cases. Formation of forsterite insulation film on grain oriented silicon steel sheets is achieved by cold rolling the sheet to the desired final sheet thickness and subjecting it to continuous annealing under wet hydrogen at a temperature of 700.degree. to 900.degree. C. This annealing functions in the following three ways to promote proper secondary recrystallization:
(1) It causes the deformed structure, after cold rolling, to recrystallize primarily; PA1 (2) It decarburizes the carbon originally present in an amount of 0.01 to 0.1% by weight in the steel sheet to a possible low of not more than 0.003% by weight; and PA1 (3) It forms an oxides layer on the surface of the steel sheet, which layer contains SiO.sub.2, a precursor of a forsterite film after oxidation, as its main component.
Subsequently to decarburization annealing an annealing separator, mainly MgO, is coated as a slurry on the steel sheet and dried; thereafter, the steel sheet is wound in a form of coil. Finishing annealing is then applied, which functions for both secondary recrystallization annealing and purification annealing. It takes place in a reducing or non-oxidizing atmosphere at a temperature not exceeding 1200.degree. C. or so. The forsterite insulation film is formed mainly by the solid phase reaction EQU 2MgO+SiO.sub.2 .fwdarw.Mg.sub.2 SiO.sub.4.
MgO is present in the annealing separator and SiO.sub.2 is present in the surface oxides layer.
The forsterite film is a thin film ceramic insulator of only several micrometers thickness, and must be very uniform and free of defects. In addition, it should have excellent adhesion to resist the forces of shearing, punching and bending, and should be smooth and have a high space factor when laminated as an iron core.
Furthermore, this forsterite film contributes to improvement of the magnetic characteristics of the sheet for reasons to be explained hereinafter. Hence, it is important to control the process of film formation to obtain excellent film quality.
The forsterite film imparts tensile stress to the steel sheet and effectively improves its iron loss and magnetostriction. Tensile stress occurs since the forsterite film undergoes less thermal expansion than the steel sheet.
The forsterite film absorbs inhibitor components which become unnecessary after completion of secondary recrystallization; this takes place during the step of high temperature annealing. This purifies the steel sheet and provides improved magnetic characteristics.
Furthermore, the formation of the forsterite film influences the inhibitor, such as MnS, MnSe or AlN, in the steel sheet during finishing annealing. Hence, this influences the secondary recrystallization itself, which is an indispensable factor in obtaining excellent magnetic characteristics of the sheet.
Formation of the forsterite film occurs at a temperature in the range of about 900.degree. C. as the temperature rises in finishing annealing. If the forsterite film forming reaction occurs too late or proceeds non-uniformly, or if the formed film is porous, oxygen and nitrogen tend to invade the steel sheet. This causes the inhibitor in the steel sheet to decompose or to turn bulky or excessive.
If the forsterite film forming reaction occurs too quickly and starts at too low a temperature, the inhibitor begins to be absorbed at a low temperature and the amount of inhibitor in the steel sheet becomes insufficient. In this way the structure of secondary recrystallization tends to have low integration of GOSS orientation and poor magnetic characteristics.
The forsterite film is a ceramic film in which fine crystals of about one micrometer size are finely integrated, and is formed on the steel sheet by use of the oxide as one raw material, as mentioned above, formed on the steel sheet surface in decarburization annealing.
The type, amount and distribution of the oxides formed on the surface layer of the steel sheet involve the formation of forsterite nuclei and growth of grains, and influence the strength of the grain boundary and the grains themselves. For example, an excessive amount of oxides formed on the surface layer of the steel sheet tends to cause local peeling of the forsterite film and to make the forsterite grains coarse. Too small an amount of oxides formed on the surface layer of steel sheet tends to form thin and brittle film some parts of which expose the bare base steel. On the other hand, an excessive amount of the oxides makes the forsterite film too thick and causes poor adhesiveness.
Increase of non-magnetic components in the steel sheet reduces the space factor when the sheet is incorporated into iron cores.
The annealing separator, which contains MgO as the main component, is coated on the steel sheet as a slurry suspended in water. Hence, the separator retains H.sub.2 O adsorbed physically even after drying. A part of the MgO is hydrated and turns to Mg(OH).sub.2. Release of H.sub.2 O therefore continues in the step of finishing annealing up to a temperature of 800.degree. C. or so, although the amount is small. The steel sheet surface is, however, oxidized by the H.sub.2 O. This oxidation phenomenon is called additional oxidation. If the extent of additional oxidation is considerable the formation rate of forsterite is restricted, and oxidation and decomposition of the inhibitor are increased in the surface layer. The secondary recrystallization grains having GOSS orientation are known to generate the nuclei and grow near the surface layer of the steel sheet. Hence, too much additional oxidation tends to deteriorate both the film characteristics and its magnetic characteristics. Susceptibility to this additional oxidation is significantly influenced by the physical properties of the oxides layer in the steel sheet surface layer that is formed in decarburization annealing.
In a grain oriented silicon steel sheet that incorporates AlN as the inhibitor, the physical properties of the oxide layer of the steel sheet influence the nitrogen removal behavior that occurs during finishing annealing. It can also influence nitrogen invasion behavior into the steel sheet from an annealing atmosphere, and therefore influences the magnetic characteristics of the sheet through the movement of the inhibitor. That is, when the nitrogen removal proceeds, the inhibiting power of the inhibitor is weakened in which case secondary recrystallization will not occur effectively and the magnetic characteristics of the sheet are caused to deteriorate. On the other hand, when nitrogen invasion becomes excessive the inhibitor becomes too strong and secondary recrystallization with good orientation hardly occurs at all.
Accordingly it is important, for the purpose of forming an excellent forsterite insulation film uniformly at a proper temperature, to control the physical properties of the oxides layer formed during decarburization annealing in the surface layer of the steel sheet. When an excellent forsterite insulation film is formed, secondary recrystallization develops under very favorable conditions. Accordingly, formation of an excellent forsterite insulation film is a very important objective in the production technology which governs the product quality of grain oriented silicon steel sheet.
In particular, in the case of thin steel sheets, the influence of the surface becomes relatively strong even in addition to the fact that the region of nuclei of the GOSS orientation becomes narrow. Control of certain physical properties of the steel sheet surface has been found to be very important for the purpose of achieving excellent magnetic characteristics.
Several processes have been proposed for improving film and magnetic characteristics by decarburization annealing of grain oriented silicon steel sheet.
JP-B 58-46547 discloses a process wherein Si, O or a silicon compound containing Si, O and H is adhered before decarburization annealing. JP-A 57-1575 discloses a process wherein the content of atmospheric components expressed as the ratio of the steam partial pressure to the hydrogen partial pressure is not less than 0.15 in the former half step of decarburization, and is not more than 0.75 in the later half step and is lower than the degree of oxidation in the early half step. JP-A 2-240215 and JP-B 54-24686 disclose processes wherein heat treatment is performed at 850.degree. to 1,050.degree. C. in a non-oxidative atmosphere after decarburization annealing.
JP-A 6-336616 discloses a process wherein the content of atmospheric components expressed as the ratio of the steam partial pressure to the hydrogen partial pressure is not more than 0.7 in the decarburization holding step, and the content of atmospheric components expressed as the ratio of the steam partial pressure to the hydrogen partial pressure in the decarburization rising step is lower than that in the decarburization holding step.
However, these processes do not give satisfactory results, although certain effects are recognized. Magnetic characteristics or adhesiveness, or coating properties or uniformity of the film have deteriorated in the width or length direction of particular steel sheet coils. Room still remains for improvement to achieve superior specifications of quality and high yield.